FIG. 1 is a cross-sectional view of an integrated circuit package. The package comprises a semiconductor die 1 on which is defined an electronic circuit. The die has a series of conductive pads 2 around its periphery. Numerous ones of the pads are attached to conductive traces 3 which extend from the die into a packaging matrix 4. The packaging matrix is formed of a non-conductive material, conventionally a polymer material. FIG. 2 is a simplified cross-section through the package on line A-A in FIG. 1. FIG. 2 shows that the conductive traces extend out of the plane of FIG. 1 so that they emerge at package connection pins 5. The package illustrated in FIGS. 1 and 2 is a QFN (quad flat no leads) package.
Some dies can be packaged in different ways for different applications. For example, a simple application that requires only some of the die's capabilities might only need to connect to 16 of the die's pads. Since that application only requires 16 pins, for that application the die can be packaged in a package whose dimensions are relatively small. Having a small package has the advantage that the device will take up less room on a circuit board. In contrast, in a more complex application it might be desired to connect to 64 of the die's pads. That would call for 64 pins, and hence a package that is large enough to accommodate 64 pins. The cost of preparing to manufacture a die is high, so it is advantageous to be able to use the same die for both these applications. Since the die will be the same in each type of packaging, it follows that the margin between the die and the periphery of the package will be smaller in the small package than in the large package. This is illustrated in FIG. 3.
FIG. 3 shows cross sections through the packaging of a common die 10 in a small package 11 and a large package 12.
FIG. 3 shows some of the traces emanating from the dies 10. In the larger package 12 many pads on the die are used. Because the margin between the die and the edge of the package is extensive, all the traces can extend at a substantial angle to the side of the die from which they emanate. In the smaller package 11 two possibilities are illustrated. On one side, at 13, the traces emanate from pads that are well spaced apart on the die. These traces all extend at substantial angles to the side of the die from which they emanate. On the other side, at 14, the traces emanate from pads that are close together on the die. Some of these traces also extend at substantial angles to the side of the die from which they emanate. However, due to the narrow margin in the smaller-sized package some of these traces must extend at lesser angles to the side of the die in order to maintain an adequate spacing between the points at which the traces terminate in the lead-out plane. For example, trace 15 extends at an angle of less than 45° to the relevant side of the die, as indicated at 16.
In chip packaging it is generally desirable for the angle at which the leads extend from the die to be as high as possible, preferably greater than 45°. Otherwise the electrical connection between the lead and its pad on the die can be unreliable or difficult to manufacture. One way to rectify the situation shown at 14 in FIG. 3 would be to physically relocate the relevant pads on the die. The die could be designed so that the pads that are to be used with the smaller chip package are separated from each other by pads that provide input/output functions that are only used in the larger-packaged device. The problem with this is that the pads that are to be used in the smaller-packaged device might provide some collective function: for example there might be four pads that together provide a UART interface. If those four pads are interspersed among other pads then when they emerge as pins in the larger-packaged device they might be spaced apart from each other even though they are to be used together. That makes it complicated to define a circuit board to which the larger package is to be connected. Of course, the leads could be re-mapped inside the package in order to rearrange them, but it is desirable to keep any rearrangement to a minimum. Indeed, it is desirable for each pin to directly overlie the end of its corresponding lead in the lead-out layer. Another option is to use different dies for the smaller and larger packages, but that would increase cost.
It is known for a die to have a multiplexer inboard of its pads in order to allow a group of input/output lines to be shifted as a block to a different set of pads. This can allow a device to adapt its pinouts to mimic another device, or to comply with a particular type of circuit board layout.
There is a need for a better way to allow a single die to be packaged in different forms.